Photochemical oxidant air pollutants such as ozone (O3) are anticipated to increase with global warming. Given the known effects of O3, this is anticipated to have major impacts on the health of children with asthma. However, there is much less information about the importance of the photochemical oxidant pollutants in secondary organic aerosols (SOA), which in many respects have the potential to do greater harm to respiratory health than ambient O3. Therefore, the potential impact on asthma morbidity among vulnerable populations from photochemical air pollutant increases resulting from global warming is likely to be underestimated using O3 alone. Vulnerable populations may include children of color and those with low socioeconomic status. Vulnerable people also include those living in heavily trafficked regions with higher concentrations of primary products of fossil fuel combustion (which include SOA precursors), or people living downwind of these source sites in hotter regions with high concentrations of photochemically aged urban aerosols including SOA. We hypothesize that daily asthma morbidity in children will be associated with increased local exposures to photochemical oxidant air pollutants, independent of the effects of primary combustion-related air pollutants. Furthermore, we hypothesize that these associations will be stronger in subjects living in regions with lower socioeconomic status and in warmer regions with more photochemical air pollution. We anticipate that such risks of asthma will increase with future changes to climate and to the underlying emissions inventory that together determine pollutant concentrations. Four aims will be used to address these hypotheses: We will first estimate daily air pollutant exposures at the zip code level throughout the state of California for 2000-2008. Air pollutants will include ambient measurements of EPA-regulated criteria air pollutants. We will also employ a source-oriented chemical transport model to estimate zip-code specific exposures to O3, NO2, size-fractionated particle species (including SOA and primary pollutants) and particle sources. We will then assess the risk of emergency department visits and hospital admissions for asthma among children from local exposures to photochemical oxidants (SOA and O3), among other pollutants. This will be followed by an assessment of effect modification of associations by subject and geographic factors that may represent increased vulnerability. Finally, we will assess the potential future risk of pediatric asthma morbidity from the effects of global warming on photochemical pollutant levels, stratified by hypothesized population vulnerabilities. The proposed research combines a state-of-the-science reactive chemical transport model with the latest SOA formation theories and applies them over climatologically relevant analysis periods to determine if SOA concentrations will experience significant changes in the future due to climate modification. Knowing this is critical to fully assessing the impact of climate change on specific populations vulnerable to the health effects of photochemical oxidant air pollution.